1. Field of the Invention
This invention relates to a vapor-liquid contacting system having particular utility in selective absorption applications.
2. Description of the Prior Art
In the art of mass transfer for selectively separating at least one component from a mixture of at least two constituents as for example in distillation and absorption applications, an upwardly flowing vapor or gas stream is typically contacted on a substantially horizontally aligned contacting surface with a generally downwardly flowing liquid stream. In a conventional absorption process, wherein a liquid solvent is employed to remove undesired constituents from the gas or vapor stream, such contacting permits the upwardly flowing vapor or gas stream to become selectively depleted in the soluble components of the gas mixture, while the generally downwardly flowing liquid stream becomes selectively enriched with the soluble constituents.
A variety of types of vapor-liquid contacting trays have been conventionally employed in the above-described mass transfer operations, to effect intimate contacting between the respective gas and liquid phases. In such applications, much effort has been expended to maximize contacting efficiency in every way possible. In these applications, tray design becomes complex because of the multiplicity of process variables and the hydraulic interrelationships between them. Contacting efficiency is related to the degree of dispersion between the respective phases to provide maximum interfacial area, adequate residence time for reaction, and good agitation in the respective phases to insure high mass transfer rates. Heretofore, high efficiency has been measured by close approaches to equilibrium, with a theoretical tray or transfer unit reflecting a 100% approach to equilibrium between the contacted phases. Close equilibrium approaches have been the goal of conventional tray design, regardless of whether the mass transfer system is gas film or liquid film controlled.
In many commercial absorption and distillation applications, the contacting operation is liquid film controlled, i.e., the liquid film resistance to mass transfer is substantially greater than the gas film resistance. In such applications, the goal of contacting tray design is to establish high liquid phase mass transfer rates. This goal generally dictates the achievement of highly agitated and dispersed liquid phase by means of the tray design. A highly agitated and dispersed liquid phase enhances contacting efficiency in liquid film controlled systems for the following reasons. First, by effecting vigorous bubbling of the liquid phase on the tray surface, the liquid phase is broken up and dispersed into thin films between adjacent gas bubbles. These bubbles are rapidly propagated and broken as gas passes upward through the tray gas flow openings and the liquid supported on the tray. In this manner, the liquid is continually agitated in a finely-divided manner. Such action yields a high degree of liquid turbulence. Secondly, a high extent of interfacial area between the respective phases is obtained by vigorous bubbling action.
In the aforementioned contacting systems wherein the contacting reaction is liquid film controlled, the vigorous bubbling action may to some extent be increased by increases in vapor velocity. The limits of tray operability and stability are quickly reached, however, since at high gas flow velocities liquid on the tray surface is sprayed into the gas space between adjacent contacting trays in the system, resulting in entrainment of the liquid in the upflowing gas. Such entrainment lowers the contacting efficiency of the trays and the contacting system as a whole.
In a contacting system where two absorption reactions are occurring simultaneously, each mass transfer rate controlled in a different phase, a most difficult problem is encountered where it is sought to selectively maximize one rate relative to another. For example, in the selective absorption of hydrogen sulfide from a gas mixture containing hydrogen sulfide and carbon dioxide by an amine liquid solvent, it is desirable to maximize the gas film limited hydrogen sulfide absorption rate, and to mimimize the liquid film limited carbon dioxide absorption rate. Inasmuch as the rate limiting step for each component in this gas mixture is known, it would be relatively simple to selectively maximize one rate relative to the other if it were possible to manipulate the mass transfer characteristics of each phase independently. The goal of the tray design for such a system, simply stated, would be to create a highly turbulent gas phase, with a relatively quiescent liquid phase. However, it is not possible to increase gas velocity in a conventionally designed contacting system so as to increase the turbulence in the gas phase without further agitating the liquid phase and improving its mass transfer properties for the undesirable liquid film limited constituent. Any increase in vapor velocity, in addition to agitating the liquid phase, increases liquid surface area and liquid residence time on the contacting tray. Furthermore, in the selective absorption of hydrogen sulfide from a gas mixture containing hydrogen sulfide and carbon dioxide, not only is it desirable to enhance gas phase mass transfer relative to liquid phase mass transfer, but it is also desirable to provide a relatively short liquid residence time so as to limit the net absorption of the undesirable component carbon dioxide. This is due to the fact that at increased carbon dioxide concentrations in the liquid phase, the mass transfer rate of hydrogen sulfide decreases. Thus, it is desirable to depart as far as possible from solution equilibrium of carbon dioxide so as to maintain maximum selectivity toward hydrogen sulfide by the solvent liquid.
For the foregoing reasons, conventional vapor-liquid contacting trays, which are designed for maximum approach to vapor-liquid contacting equilibrium, perform poorly in applications where it is desired to maximize the absorption of a gas film limited constituent while minimizing the absorption of a liquid film limited constituent.
Accordingly, it is an object of the present invention to provide an improved vapor-liquid contacting tray for selectively absorbing a constituent from a gas mixture which is gas film limited relative to an undesired constituent of the gas mixture which is liquid film limited.
It is another object of the present invention to provide an improved process for absorbing hydrogen sulfide from a gas mixture containing hydrogen sulfide and carbon dioxide by an amine liquid solvent in an absorption zone containing a plurality of generally horizontally aligned, vertically spaced-apart perforated trays on which liquid solvent, flowing generally downwardly through the adsorption zone from tray to tray, is contacted with upwardly flowing gas mixture.
Other objects and advantages of the present invention will be apparent from the ensuing disclosure and appended claims.